The present invention relates to an image-data interpolation processing method for interpolation of insufficient image data to achieve high-resolution transformation and magnification of a multigradational image containing different kinds of components coexisting in the image.
A typical image-processing device for processing an image data inputted from a scanner or the like input device usually conducts interpolation of insufficient image data to convert a low-resolution multigradational image into a high-resolution and/or an enlarged small-sized multigradational image. There are several known methods for interpolating image data. Examples are nearest neighbor interpolation that directly applies data of the nearest neighbor to a pixel to be interpolated, bilinear interpolation that calculates a planar product-sum from surrounding pixels and cubic convolution interpolation that calculates a curved-surface product-sum according to the surrounding pixels.
These interpolation processing methods have both merits and demerits. The nearest neighbor interpolation has the shortest processing time but produces jaggies of an oblique line, resulting in deterioration of the image. The bilinear interpolation has a relatively short processing time and suitable to interpolate a portion having a smooth change in density, but it causes unclearness of a portion sharply changing in density such as an edge of the image. The cubic convolution interpolation can smoothly interpolate a portion having a moderate change in density with a small deterioration of the image quality and clearly reproduce an edge portion but it requires a long time of processing and emphasizes a noise existing in a smooth density-varying portion, resulting in the deterioration of the image quality.
Consequently, an image that includes a character (edge-like) image area and photographic (non-edge) image area cannot sufficiently be processed by applying merely one of the above methods of interpolation. In other words, any of the above interpolation methods cannot convert and/or enlarge the image in such a way that it may realize at the same time the high-resolution of the character-image area and the smoothly varying density of the photographic image area.
Accordingly, an attempt has been made to provide an image processing device that strictly distinguishes an edge-component area from an edgeless area and processes these different areas by different methods of interpolation. For example, Japanese Laid-Open Patent Publication No. 5-135165 discloses such an arrangement that determines the maximum value and the minimum value of densities in a local area including attentional pixel and surrounding pixels, discriminates, based on the maximal density value obtained by subtracting the least value from the largest value, whether the attentional pixel belongs to the character (edge) area or the (non-edge) face-area, and executes processing adapted to the character image area or the photographic image area.
However, the image processing device disclosed in Japanese Laid-Open Patent Publication No. 5-135165, which extracts an edge image area according to a change in density of pixels including an attentional pixel, involves following problems:
The device may obtain an erroneous large differential density value from a partial image area containing a noise irrespective of the area having a moderate variation of densities of pixels including an attentional pixel.
Furthermore, the rotation of a whole image may cause a change in a density pattern of the partial image area due to the changed direction of the edge image. In other words, the same partial image area extracted in different directions may have two different discrimination results. To avoid this, different extracting conditions must be provided for the image before and after rotated, whereby the discrimination processing may be complicated.
In addition, image areas must be separated before interpolation, whereby the processing not only takes longer time but also complicates the circuitry of the device which, therefore, has an enlarged size and is expensive to manufacture.